1. Field of the Invention
The present invention relates to an a-Si TFT liquid crystal device, a liquid crystal display panel and a method of manufacturing them.
2. Description of Related Art
A conventional method for manufacturing an a-Si TFT liquid crystal display panel will now be described with regard to one liquid crystal device portion. As shown in FIGS. 15(a) and 15(b), a gate electrode 4 is first formed into a predetermined shape on a glass substrate 2 by a general technique. Then, as shown in FIG. 16, a gate insulating film 6, a channel layer 8 and a SiNx film 10 serving as an etching stopper layer are successively stacked on the entire surface of the glass substrate 2. As shown in FIGS. 17(a), 17(b) and 17(c), after coating a resist on the SiNx film 10, the glass substrate 2 is exposed to light from its back surface side by using the gate electrode 4 in the predetermined shape as a light shielding mask, and is subsequently subjected to stepper exposure from its front surface side by using a reticle, and then the resist is developed. Thereafter, the SiNx film 10 other than a portion working as an etching stopper (channel protecting film) 14 is etched with diluted hydrofluoric acid, and then the resist is removed.
It is noted that the etching stopper 14 can be basically formed through a single exposing step in which the glass substrate 2 is subjected to the stepper exposure from its front surface side by using a reticle. In this conventional manufacturing method, however, the etching stopper 14 is formed through a two-stage exposing step: a stage of exposing the substrate 2 to light from its back surface side and a stage of exposing the substrate 2 to light from its front surface side. This is because, when the etching stopper 14 is formed through a single-stage exposing step in which the glass substrate 2 is exposed to light from its front surface side, the alignment with the gate electrode 4 tends to be shifted and cannot be stabilized. In contrast, if the gate electrode 4 is effectively used in the two-stage exposing step, the etching stopper 14 can be disposed at the center of the gate electrode 4 in a self-alignment manner. This results in providing a source electrode 26 and a drain electrode 28 symmetrically about the gate electrode 4, and also in reducing overlap areas between the gate electrode 4 and the drain electrode 28 and between the gate electrode 4 and the source electrode 26. Thus, the two-stage exposure can improve a transistor characteristic.
However, in manufacturing a liquid crystal display panel, there are a large number of complicated manufacturing steps, and in addition, each steps requires time-consuming work. Therefore, reduction in the number of processes not only improves the productivity but also reduces the manufacturing cost of a liquid crystal display panel in which the process cost accounts for a large proportion.
After being developed, the resist used for forming the etching stopper 14 has a size of approximately 20xc3x9710 xcexcm per pixel, and such rectangular resists are arranged side by side over the entire surface of an array substrate. Since the area of each resist is thus small, its adhesion to the underlying nitride film (i.e., the SiNx film) is low, and hence, the resist is apt to be easily peeled off. When the resist is peeled off, the etching stopper 14 cannot be properly formed, which leads to a transistor failure.
When the etching stopper layer 10 is etched with diluted hydrofluoric acid, the layer 10 is generally over-etched so as not to leave an insufficiently etched portion. However, excessive over-etching makes the side surface of the etching stopper 14 be inclined inward at the foot thereof, resulting in the formation of a xe2x80x9cconcavexe2x80x9d 15, which is hidden in a top view as shown in FIG. 17(c). When films and/or foreign matter to be deposited in subsequent steps are attached to the concave 15, they cannot be removed through cleaning and etching. As a result, as shown in FIG. 18, a leakage current flows between the source electrode 26 and the drain electrode 28 formed on the etching stopper 14, which leads to a leakage failure of the transistor.
As the result of researches to remove the above disadvantages, the present inventors have eventually found the present invention. An object of the present invention is to reduce the number of steps, especially the number of exposing steps in manufacture of a liquid crystal device, so as to improve the productivity and reduce the manufacturing cost.
Another object of the present invention is to prevent a failure of a transistor related to an etching stopper portion, so as to improve the yield and the quality of a liquid crystal display panel.
The liquid crystal device of the present invention comprises an etching stopper whose two pairs of opposite side surfaces are inclined at different angles. Specifically, the two pairs of opposite side surfaces of the etching stopper of the liquid crystal device are respectively etched in different steps, and hence, they are generally inclined at different angles.
Alternatively, the liquid crystal device of the present invention comprises an etching stopper, one of opposite side surfaces of which is at substantially right angles to the substrate or tapers away from the substrate. When a side surface extending along current flow is oppositely tapered, a leakage current can be caused owing to a remaining impurity and the like. Therefore, such a side surface is desired to be at right angles or normally tapered. On the other hand, when a side surface extending perpendicularly to the current flow (namely, a side surface covered with a source electrode and a drain electrode) is oppositely tapered and is not controlled, the overlap area between the gate electrode and the source (or drain) electrode can be varied, resulting in varying the parasitic capacitance of each device. Accordingly, such a side surface is preferably controlled to be at right angles or to be normally tapered. The side surface extending perpendicularly to the current flow can be also formed into a normally tapered shape through wet etching in the present invention because the covering area of the etching stopper can be large so as to reduce the over-etching amount attained when the etching is almost completed.
In the liquid crystal device of the present invention, at least one side surface of an etching stopper extending perpendicularly to current flow, namely, at least one side surface not covered with the source electrode or the drain electrode, is at substantially right angles to the substrate or tapers away from the substrate. Accordingly, no attachment such as an insufficiently etched film and an impurity remains on the etching stopper portion, resulting in preventing a leakage current from flowing between the source electrode and the drain electrode.
Alternatively, the method of manufacturing a liquid crystal display panel of the present invention comprises at least a step of simultaneously etching an etching stopper and a source/drain layer. This manufacturing method makes it possible to eliminate a concave of the etching stopper, which is formed in the channel width direction as a result of excessive over-etching, and hence, a leakage current can be prevented from flowing between the source electrode and the drain electrode.
Moreover, in the alternative method of manufacturing a liquid crystal display panel according to the present invention, a gate insulating film, a channel layer and an etching stopper layer are formed on a transparent substrate bearing a gate electrode, and the substrate is exposed to light from its back surface side by using the gate electrode as a light shielding mask by a photography technique. Then, the resist is developed, and the etching stopper layer is etched, and thereby an etching stopper is formed. This manufacturing method makes it possible to form the etching stopper through an only sigle-stage exposing step. As a result, the productivity can be largely improved, and the occurrence of formation failures of the etching stopper derived from a resist failure can be substantially avoided.
Moreover, in the method of manufacturing a liquid crystal display panel, after the aforementioned steps, a source/drain layer is formed. Then, the source/drain layer and a remaining portion of the etching stopper are etched through chemical gas phase etching by photolithography. This manufacturing method makes it possible to eliminate a concave of the etching stopper, which is formed in the channel width direction as a result of excessive over-etching, and therefore, a leakage current can be prevented from flowing between the source electrode and the drain electrode.
In the liquid crystal device, the liquid crystal display panel and the method for manufacturing them according to the present invention, exposure of an etching stopper layer includes merely one exposure process using a gate electrode as a light shielding mask. Thus, the number of exposure processes in this invention is smaller by one than that of the conventional technique. Since the method of the present invention does not include the exposure process using a light-shielding mask such as a reticle, which requires preciseness in alignment and takes a long time, the productivity can be remarkably improved.
In addition, a resist for forming the etching stopper is formed on the entire gate electrode and has a larger adhesion area than that used in the conventional technique. Therefore, the photoresist is less likely to peel off, and hence, the occurrence of formation failures of the resist can be suppressed, and the failure of transistors can be reduced. Furthermore, the manufacturing cost can be largely decreased because an expensive reticle is not necessary.
In the manufacturing method of the present invention, an etching stopper which is longer in the channel width direction than the width of a source/drain electrode under layer is formed through an exposure process from the back surface side of a substrate and an exposure process from the front surface thereof. Therefore, a reticle and another light-shielding mask used in the exposure process from the front surface of the substrate can be merely roughly aligned. As a result, not only the workability involved in the alignment can be remarkably improved but also the occurrence of failures can be largely reduced and the device quality can be stabilized.
Furthermore, the etching stopper longer in the channel width direction than the width of the source/drain electrode under layer is removed at its ends through etching simultaneously with the formation of the source/drain electrode under layer. Therefore, a resist used for forming the source/drain electrode under layer can be very easily aligned, and the occurrence of failures involved in the alignment can be substantially avoided.
Moreover, since the etching stopper longer in the channel width direction is etched at its ends simultaneously with the formation of the source/drain electrode under layer, even when over-etching in the etching of the etching stopper layer is so excessive that a concave in an oppositely tapered shape is formed, the concave extending over a source electrode and a drain electrode can be removed through etching. As a result, no leakage current flows between the source electrode and the drain electrode, and the performance and the quantity can be improved. In addition, since the concave formed through the excessive over-etching can be thus removed, resultant products can be used as good products. Thus, the occurrence of failure can be largely reduced.